In the related art, a transmission device is known which uses a transmission unit provided with a sensor to transmit detection values of physical states, such as temperature, humidity, or pressure, detected using a sensor to a separate location, via radio waves.
For example, a transmission device is known which is able to monitor the air pressure of an inner section in a pneumatic fender, which is used when a ship is moored to a quay on a shore or another ship, without touching the pneumatic fender by detecting the air pressure of an inner section using a sensor of a transmission unit, using the transmission unit to transmit the detected value to a separate location via radio waves, and receiving the detected value.
As an example of a system which uses a fender provided with such a sensor, a ship maneuvering and ship mooring support system disclosed in WO2008/053887 JP is known. In this system, when a plurality of fenders are attached to a side surface of a ship and two ships are moored to each other, it is possible to maneuver a ship while monitoring the internal air pressure of the fenders. For example, in a case where four fenders 1A, 1B, 1C, and 1D are attached to a side surface of a ship Sp1 as illustrated in FIG. 17 to FIG. 19, transmission units 100 stored in a case 11 of a transmission device 10 is provided in inner sections of each of the fenders 1A, 1B, 1C, and 1D, the air pressure of the inner sections of the fenders is detected by sensors provided in the transmission units 100 and the information on the detected air pressure is transmitted as digital data. The data which is transmitted from each of the fenders 1A, 1B, 1C, and 1D is received by a reception antenna 2 which is attached to the side of a control room. Here, the distances between each of the fenders 1A, 1B, 1C, and 1D and the reception antenna 2 are respectively La, Lb, Lc, and Ld (m).
The ship Sp1 and the fenders 1A, 1B, 1C, and 1D float on the ocean, and it goes without saying that the sea surface moves up and down according to the waves. For this reason, as illustrated in FIG. 20, a distance Hs between transmission units 100 of each of the fenders 1A, 1B, 1C, and 1D and the sea surface changes according to up and down changes Hv in the sea surface and, in accordance with this change, the strength of the received radio waves from each of the fenders 1A, 1B, 1C, and 1D in the reception antenna 2 is greatly changed. That is, in the radio waves radiated from the transmission units 100 provided in the fenders 1, there are waves which reach the reception antenna 2 as direct waves DW, waves which reach the reception antenna 2 as reflected waves RW1a, RW1b, RW2, and RW3, and waves which do not reach the reception antenna 2. For example, there are cases where the reflected waves which include the reflected wave RW1a reflected off a sea surface 5 at the lowest point and the reflected wave RW1b reflected off a highest sea surface 5a reach the reception antenna 2 and cases where the reflected waves do not reach the reception antenna 2. Further, when the reflected waves reach the reception antenna 2, the reception electric field strength may be increased or decreased due to interference with the direct waves DW. In addition, the reflected wave RW2 which is reflected off a floating object 6 present in seawater 4 does not reach the reception antenna 2 due to being attenuated by the seawater, and the reflected wave RW3 which is reflected off a seabed 3 also does not reach the reception antenna 2 due to being attenuated by the seawater.
FIG. 21 to FIG. 24 show measurement values of the reception electric field strength from each of the fenders 1A, 1B, 1C, and 1D at this time. In the drawings, the curve D is a distance between the mooring and the ship Sp1, and RSSI is the reception electric field strength. As shown in these drawings, the reception electric field strength from each of the fenders 1A, 1B, 1C, and 1D greatly changes according to changes in the height of the sea surface due to the waves, the reception electric field strength from each of the fenders 1A, 1B, 1C, and 1D may fall below the reception limit strength in the reception antenna 2, and it may not be possible to receive the data.